1. Field of the Invention
The present invention relates generally to a scramjet combustor for a hypersonic flight vehicle, and, more particularly, to a scramjet combustor having a two-part, aft-facing step with primary and secondary fuel injector discharge orifices for improved combustor performance over a wide range of hypersonic Mach flight numbers.
2. Description of Related Art
Hypersonic flight vehicles have been proposed which incorporate scramjet engines to achieve high Mach numbers. Once such a vehicle has achieved a sufficient speed by some other propulsive means (which may include a turbojet engine), a scramjet engine will take over to propel the aircraft to high Mach numbers (typically between Mach 6 and Mach 20). Such high Mach numbers cannot be achieved by any other known type of air-breathing engine.
A typical scramjet engine includes a combustor having a chamber, wherein a fuel-air mixture moving at supersonic speed is burned, and having at least one fuel injector which directs supersonically-moving fuel (such as pressurized hydrogen) into the chamber. The engine also includes an air inlet, which delivers compressed supersonically-moving air to the combustor chamber, and further includes an exhaust nozzle, which channels the burning gases out of the combustor chamber to help produce engine thrust. The fuel injector discharge orifices are the openings in the combustor chamber to which fuel is delivered by a fuel system which may include tanks, pumps, and conduits. In the case hydrogen fuel, the fuel-air mixture in the combustor chamber will have a high enough temperature and pressure to auto-ignite.
Known scramjet combustor designs include combustor walls having an aft-facing step and include an angled fuel injector located at the top of the step or a transversely-directed fuel injector located at the bottom of the step. Typically, known fuel injectors appear to be round conduits. "Angled" fuel injection means the injected fuel is not parallel or perpendicular (transverse) to the generally longitudinally-moving air. It is known in the art that as the flight Mach number increases above about Mach 10-12, a substantial and increasing portion of the engine thrust comes from the discharge of the pressurized fuel from the angled fuel injectors and not from the burning of that fuel. It is also known that angled or transverse fuel injection promotes significantly better fuel penetration into the airstream and better fuel-air mixing than can be achieved from axial injection. One scramjet combustor design which achieves adequate fuel penetration across the height of the combustion chamber with axial injection, without utilizing some type of fuel injector which protrudes into the airstream and thereby produces large thrust losses and increased fuel cooling requirements, is disclosed in U.S. Pat. No. 5,349,815, entitled "Scramjet Combustor Having a Two-Part, Aft-Facing Step" by Paul H. Kutschenreuter, Jr.
U.S. Pat. No. 5,349,815, which is hereby incorporated by reference, discloses a flight vehicle scramjet combustor having two spaced-apart, generally opposing, and generally longitudinally extending walls extending forward and aft. One of the walls includes a generally aft-facing step, a forward wall portion extending generally longitudinally forward of the step, and an aft wall portion extending generally longitudinally aft of the step. The step includes a first section and an interconnected second section. The first section is attached to the forward wall portion, while the second section is attached to the aft wall portion. The second section includes a fuel injector discharge orifice having a fuel discharge axis. The fuel discharge axis is aligned generally perpendicular to the second section, projects generally towards the other wall, and projects generally longitudinally aft. The fuel discharge orifice preferably has a generally rectangular shape with its two shorter sides generally aligned with the intersection of the first and second sections. Where the one wall is generally planer, the intersection of the forward wall portion and the first section preferably is a generally straight line having one end positioned longitudinally forward of its other end.
The scramjet combustor design of U.S. Pat. No. 5,349,815, provides several benefits and advantages over the prior art scramjet combustors, including excellent penetration and mixing of hydrogen fuel at scramjet flight Mach numbers as high as Mach 18 while recovering a significant amount of the axial fuel momentum without incurring intrusive drag losses. This is accomplished principally by the use of a large number of small, closely spaced narrow fuel injector discharge orifices or slots in the slanted step. However, it has been found that such a design is not as effective at lower flight Mach numbers (such as Mach 6-Math 9) since decreased inlet contraction ratios typically increase the combustor passage height on the order of 50%. Also, because decreased fuel cooling requirements permit significant reductions in fuel equivalence ratios compared to higher Mach number scramjet operation, the net result is that lower flight Mach number scramjet operation involves getting less fuel per unit airflow across a larger combustor passage height. Therefore, a scramjet combustor is needed which utilizes the two-part aft-facing step of U.S. Pat. No. 5,349,815, but is also able to operate more effectively over a wider range of hypersonic Mach flight numbers.